Steering control device

ABSTRACT

A steering control device is provided that calculates a white line approach suppression steering reaction force that is based on a distance in which a host vehicle is separated from either a left or a right white line when the host vehicle approaches the left or a right white line. The steering control device applies a steering reaction force to a steering unit that receives steering input from a driver, based on the white line approach suppression steering reaction force, and upon determining that the other among the left and right white lines cannot be detected and a distance between the host vehicle and the left or right white line is decreasing, the white line approach suppression steering reaction force is limited.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National stage application of InternationalApplication No. PCT/JP2013/075801, filed Sep. 25, 2013, which claimspriority to Japanese Patent Application No. 2012-221785 filed in Japanon Oct. 4, 2012.

BACKGROUND

1. Field of the Invention

The present invention relates to a steering control device.

2. Background Information

Japanese Laid-Open Patent Application No. 2011-051570 discloses asteering control device for assisting the steering of the driver, whenthe host vehicle approaches either a left or a right white line of thetraveling lane that is recognized based on an image of an onboardcamera, by turning the turnable wheel in a direction that will move thehost vehicle away from the white line.

SUMMARY

In the conventional technology described above, when the other among theleft and right white lines (that which is farther from the host vehicle)becomes undetectable during the implementation of the control to assistthe steering of the driver, continued control will not immediatelybecome impossible because the control amount of the control forassisting the steering of the driver is based on the relationship witheither the left or right white lines; however, since, conceivably, thehost vehicle will subsequently approach the other among the left andright white lines, interrupting the control for assisting the steeringof the driver is necessary even in a case in which the white linefarther from the host vehicle becomes undetectable. Here, if the controlis simply interrupted, the steering reaction force will be contrary towhat the driver expects because the driver is steering while assuming achange in the steering reaction force generated by the control,imparting discomfort to the driver. The object of the present inventionis to provide a steering control device that is capable of reducing thisdiscomfort imparted to the driver.

When a host vehicle approaches either a left or a right white line, awhite line approach suppression steering reaction force that increasesas the distance between the host vehicle and the white line decreases iscalculated; when applying a steering reaction force to the steering unitthat receives a steering input from the driver based on the white lineapproach suppression steering reaction force, if a determination is madethat the other among the left and right white lines cannot be detectedand an increase gradient of the white line approach suppression steeringreaction force becomes equal to or less than a predetermined increasegradient, the white line approach suppression steering reaction force islimited.

Thus, by initiating a limit on the white line approach suppressionsteering reaction force at a point in time when an increasing tendencyof the white line approach suppression turning amount ends, suppressingthe steering reaction force from deviating from the expectation of thedriver is possible, and the discomfort imparted to the driver can bereduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the attached drawings which form a part of thisoriginal disclosure.

FIG. 1 is a system view illustrating the steering system of a vehicle ofthe first embodiment.

FIG. 2 is a control block view of the turning control unit 19.

FIG. 3 is a control block view of a steering reaction force control unit20.

FIG. 4 is a control block view of a external disturbance suppressioncommand turning angle calculation unit 32.

FIG. 5 is a control block view of a repulsive force calculation unit 37corresponding to the yaw angle.

FIG. 6 is a control block view of a repulsive force calculation unit 38corresponding to the lateral position.

FIG. 7 is a view illustrating a control region of the yaw angle F/Bcontrol and the lateral position F/B control.

FIG. 8 is a time chart illustrating the yaw angle change when a vehicletraveling a straight road of a highway receives a sporadic crosswind.

FIG. 9 is a time chart illustrating the yaw angle change and the lateralposition change when the lateral position F/B control is not executedwhen a vehicle traveling a straight road of a highway receives acontinuous crosswind.

FIG. 10 is a time chart illustrating the yaw angle change and thelateral position change when the lateral position F/B control isexecuted when a vehicle traveling a straight road of a highway receivesa continuous crosswind.

FIG. 11 is a control block view of a lateral force offset unit 34.

FIG. 12 is a view illustrating a state in which a steering reactionforce characteristic, representing the steering reaction force torquecorresponding to a self-aligning torque, is offset in the same directionas the self-aligning torque.

FIG. 13 is a characteristic view illustrating the relationship betweenthe steering angle of the steering wheel and the steering torque of thedriver.

FIG. 14 is a view illustrating a state in which a characteristicillustrating the relationship between the steering angle of the steeringwheel and the steering torque of the driver has been changed byoffsetting the steering reaction force characteristic, representing thesteering reaction force torque corresponding to the self-aligningtorque, in the same direction as the self-aligning torque.

FIG. 15 is a control block view of a steering reaction force torqueoffset unit 36.

FIG. 16 is a control block view of a reaction force calculation unit 39corresponding to the deviation margin time.

FIG. 17 is a control block view of a reaction force calculation unit 40corresponding to the lateral position.

FIG. 18 is a view illustrating a state in which the steering reactionforce characteristic, representing the steering reaction force torquecorresponding to the self-aligning torque, is offset in a direction inwhich the absolute value of the steering reaction force torque becomeslarger.

FIG. 19 is a characteristic view illustrating the relationship betweenthe steering angle of the steering wheel and the steering torque of thedriver.

FIG. 20 is a view illustrating a state in which the characteristicillustrating the relationship between the steering angle of the steeringwheel and the steering torque of the driver has been changed byoffsetting the steering reaction force characteristic, representing thesteering reaction force torque corresponding to the self-aligningtorque, in a direction in which the absolute value of the steeringreaction force torque becomes larger.

FIG. 21 is a system view illustrating the steering system of a vehicleof a second embodiment.

FIG. 22 is a control block view of an assist torque control unit 28.

FIG. 23 is a control block view of an assist torque offset unit 42.

FIG. 24 is a view illustrating a state in which the assist torquecharacteristic, representing the assist torque corresponding to thesteering torque, is offset in a direction in which the absolute value ofthe assist torque becomes smaller.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is a system view illustrating a steering system of a vehicle ofthe first embodiment.

The steering device of the first embodiment is mainly configured by asteering unit 1, a turning unit 2, a backup clutch 3, and an SBWcontroller 4, and the device employs a steer-by-wire (SBW) system inwhich the steering unit 1 that receives steering input from a driver andthe turning unit 2 that turns a left and a right front wheel (theturnable wheels) 5FL, 5FR are mechanically separated.

The steering unit 1 is provided with a steering wheel 6, a column shaft7, a reaction force motor 8, and a steering angle sensor 9. The columnshaft 7 rotates integrally with the steering wheel 6. The reaction forcemotor 8 is, for example, a brushless motor, and a coaxial motor in whichthe output shaft is coaxial with the column shaft 7 outputs a steeringreaction force torque to the column shaft 7 in response to a commandfrom the SBW controller 4. The steering angle sensor 9 detects theabsolute rotation angle of the column shaft 7, that is, the steeringangle of the steering wheel 6.

The turning unit 2 is provided with a pinion shaft 11, a steering gear12, a turning motor 13, and a turning angle sensor 14. The steering gear12 is a rack-and-pinion-type steering gear, which turns the front wheels5L, 5R in response to the rotation of the pinion shaft 11. The turningmotor 13 is, for example, a brushless motor, in which the output shaftis connected to a rack gear 15 via an unillustrated decelerator, andthis motor outputs a turning torque for turning the front wheel 5 to arack 16 in response to a command from the SBW controller 4. The turningangle sensor 14 detects the absolute rotation angle of the turning motor13. Since there is an always uniquely determined correlation between therotation angle of the turning motor 13 and the turning angle of thefront wheel 5, the turning angle of the front wheel 5 can be detectedbased on the rotation angle of the turning motor 13. Herein below,unless specifically described, the turning angle of the front wheel 5shall be that which is calculated based on the rotation angle of theturning motor 13. The backup clutch 3 is provided between the columnshaft 7 of the steering unit 1 and the pinion shaft 11 of the turningunit 2, and the steering unit 1 and the turning unit 2 are detached by arelease; the steering unit 1 and the turning unit 2 are mechanicallyconnected by fastening thereof.

The vehicle speed (the vehicle body speed) detected by an image of thetraveling road in front of the host vehicle captured by a camera 17 anda vehicle speed sensor 18, in addition to the steering angle sensor 9and the turning angle sensor 14 described above, are input into the SBWcontroller 4. The SBW controller 4 comprises a turning control unit 19for controlling the turning angle of the front wheels 5FL, 5FR, asteering reaction force control unit 20 for controlling the steeringreaction force torque applied to the column shaft 7, and an imageprocessing unit 21. The turning control unit 19 generates a commandturning angle based on each piece of input information and outputs thegenerated command turning angle to an electric current driver 22. Theelectric current driver 22 controls the command electric current to theturning motor 13 by an angle feedback for matching the actual turningangle detected by the turning angle sensor 14 with the command turningangle. The steering reaction force control unit 20 generates a commandsteering reaction force torque based on each piece of input informationand outputs the generated command steering reaction force torque to anelectric current driver 23. The electric current driver 23 controls thecommand electric current to the reaction force motor 8 by a torquefeedback for matching the actual steering reaction force torque that isinferred from the current value of the reaction force motor 8 with thecommand steering reaction force torque. The image processing unit 21recognizes the traveling lane left and right white lines (the travelingpath dividing lines) by image processing, such as by edge extractionfrom an image of a traveling path in front of a host vehicle captured bya camera 17. In addition, when the SBW system fails, the SBW controller4 fastens the backup clutch 3 and mechanically couples the steering unit1 and the turning unit 2, allowing the rack 16 to move in the axialdirection by steering the steering wheel 6. At this time, a controlcorresponding to an electric power steering system for assisting thesteering force of the driver by an assist torque of the turning motor 13can be executed. The SBW system described above may be a redundantsystem provided with a plurality of each sensor, each controller, andeach motor. Additionally, the turning control unit 19 and the steeringreaction force control unit 20 may be separate bodies.

In the first embodiment, stability control and corrective steeringreduction control are executed with the aim of reducing the correctivesteering amount and reducing the burden of steering for the driver.Stability control aims to improve the safety of a vehicle with respectto disturbance (crosswind, uneven road surfaces, ruts, road surfacecants, etc.) and performs two feedback (F/B) controls.

1. Yaw Angle F/B Control

The yaw angle generated by disturbance is reduced by correcting theturning angle in accordance with the yaw angle, which is the anglebetween the white line and the host vehicle traveling direction.

2. Lateral Position F/B Control

The lateral position change, which is the integrated value of the yawangles generated by disturbance, is reduced by correcting the turningangle in accordance with the distance to the white line (the lateralposition).

The corrective steering reduction control aims to improve safety of avehicle with respect to steering input from the driver and performsthree reaction force offset controls.

1. Reaction Force Offset Control Corresponding to the Lateral Position

The steering reaction force characteristic corresponding to theself-aligning torque is offset in a direction in which the absolutevalue of the steering reaction force becomes larger in accordance withthe lateral position in order to suppress the sign of the steeringtorque from being inverted when a driver performs corrective steeringthat straddles the steering angle neutral position.

2. Reaction Offset Control Corresponding to the Deviation Margin Time.

The steering reaction force characteristic corresponding to theself-aligning torque is offset in a direction in which the absolutevalue of the steering reaction force becomes larger in accordance withthe deviation margin time (the time required to reach the white line) inorder to suppress the sign of the steering torque from being invertedwhen a driver performs corrective steering that straddles the steeringangle neutral position.

3. Reaction Force Offset Control Corresponding to the Curvature.

The steering reaction force characteristic corresponding to theself-aligning torque is offset in the same coding direction as theself-aligning torque in accordance with the curvature of the white linein order to reduce the steering retention force of the driver and tosuppress a change in the steering retention angle with respect to achange in the steering retention force when turning.

Turning Control Unit

FIG. 2 is a control block view of the turning control unit 19. The SBWcommand turning angle calculation unit 31 calculates an SBW commandturning angle based on the steering angle and vehicle speed. Theexternal disturbance suppression command turning angle calculation unit32 calculates a external disturbance suppression command turning anglefor correcting the SBW command turning angle during stability control,based on vehicle speed and the white line data. The details of theexternal disturbance suppression command turning angle calculation unit32 will be described below. The adder 19 a adds the SBW command turningangle and the external disturbance suppression command turning angle andoutputs the value obtained to the electric current driver 22 as thefinal command turning angle.

Steering Reaction Force Control Unit

FIG. 3 is a control block view of a steering reaction force control unit20. The lateral force calculation unit 33 calculates a tire lateralforce by referencing a steering angle-lateral force conversion maprepresenting the relationship between the steering angle and the tirelateral force per vehicle speed in a conventional steering device, whichhas been obtained by experimentation or another means beforehand, basedon the steering angle and the vehicle speed. The steering angle-lateralforce conversion map has a characteristic in which the tire lateralforce increases as the steering angle increases; the change amount ofthe tire lateral force with respect to the change amount of the steeringangle is larger when the steering angle is small, as compared to whenlarge; and the tire lateral force becomes smaller as the vehicle speedincreases. The lateral force offset unit 34 calculates a lateral forceoffset amount for offsetting the steering reaction force characteristicin a reaction force offset control corresponding to the curvature, basedon vehicle speed and the white line data. The details of the lateralforce offset unit 34 will be described below. The subtracter 20 asubtracts the lateral force offset amount from the tire lateral force.The SAT calculation unit 35 calculates a steering reaction force torquethat is generated by the tire lateral force by referencing a lateralforce-steering reaction force torque conversion map, representing therelationship between the tire lateral force and the steering reactionforce torque in a conventional steering device obtained byexperimentation or another means beforehand, based on vehicle speed andthe tire lateral force after an offset by the lateral force offsetamount. The tire lateral force-steering reaction force torque conversionmap has a characteristic in which the steering reaction force torque islarger as the tire lateral force increases; the change amount of thesteering reaction force torque with respect to the change amount of thetire lateral force is larger when the tire lateral force is small, ascompared to when large; and the steering reaction force torque becomessmaller as the vehicle speed increases. This characteristic simulates areaction force that is generated in the steering wheel by theself-aligning torque of wheels trying to return to a straight state,which is generated by the road surface reaction force in a conventionalsteering device. The adder 20 b adds a steering reaction force torquecomponent (a spring item, a viscous item, an inertia item) correspondingto the steering reaction force torque and the steering characteristic.The spring item is a component that is proportional to the steeringangle and is calculated by multiplying a predetermined gain and thesteering angle. The viscous item is a component proportional to thesteering angular velocity and is calculated by multiplying apredetermined gain and the steering angular velocity. The inertia itemis a component that is proportional to the steering angular accelerationand is calculated by multiplying a predetermined gain and the steeringangular acceleration. The steering reaction force torque offset unit 36calculates a steering reaction force torque offset amount for offsettingthe steering reaction force characteristic in a reaction force offsetcontrol corresponding to the lateral position or the deviation margintime, based on the vehicle speed and the image of a traveling path infront of the host vehicle. The details of the steering reaction forcetorque offset unit 36 will be described below.

The reaction force offset control suppression unit 43 comprises acounter 43 a and a steering reaction force torque offset amount limitingunit 43 b. When the white line farther from the host vehicle among theleft and right white lines becomes undetectable, the counter 43 a startsto count up; when the count value reaches a threshold value, the countvalue is reset (=0), and a reaction force offset control suppressionflag is set (=1). The reaction force offset control suppression flag isreset (=0) when the white line becomes undetectable. The steeringreaction force torque offset amount limiting unit 43 b starts a reactionforce offset control suppression operation when a reaction force offsetcontrol suppression flag is set and the steering reaction force torqueoffset amount is below the previous value. In the reaction force offsetcontrol suppression operation, the steering reaction force torque offsetamount is limited. Specifically, when the steering reaction force torqueoffset amount exceeds the previous value, the previous value is outputas the steering reaction force torque offset amount, and the currentvalue is output when equal to or greater than the previous value. Theadder 20 c outputs a value obtained by adding the steering reactionforce torque, after adding a steering reaction force torque componentcorresponding to the steering characteristic, and the steering torqueoffset amount to the electric current driver 23 as the final commandsteering reaction force torque.

External Disturbance Suppression Command Turning Angle Calculation Unit

FIG. 4 is a control block view of a external disturbance suppressioncommand turning angle calculation unit 32. The yaw angle calculationunit 32 a calculates the yaw angle, which is an angle between the whiteline in a forward fixation point and the traveling direction of the hostvehicle. The yaw angle at the forward fixation point shall be the angleformed between the white line after a predetermined time (for example,0.5 seconds) and the traveling direction of the host vehicle. The yawangle can be easily and precisely detected by calculating the yaw anglebased on an image of the traveling path captured by the camera 17. Thecurvature calculation unit 32 b calculates the curvature of the whiteline at the forward fixation point. A lateral position calculation unit32 c calculates the distance to the white line at the forward fixationpoint. The repulsive force calculation unit 37 corresponding to the yawangle calculates the repulsive force of the vehicle for reducing the yawangle that is generated by disturbance in a yaw angle F/B control, basedon the yaw angle, the curvature, and the vehicle speed. The details ofthe repulsive force calculation unit 37 corresponding to the yaw anglewill be described below.

The repulsive force calculation unit 38 corresponding to the lateralposition calculates the repulsive force of the vehicle for reducing thelateral position change that is generated by disturbance in a lateralposition F/B control, based on the yaw angle, the curvature, the vehiclespeed, and the distance to the white line at the forward fixation point.The details of the repulsive force calculation unit 38 corresponding tothe lateral position will be described below. The adder 32 d adds arepulsive force corresponding to the yaw angle and a repulsive forcecorresponding to the lateral position and calculates the lateraldirection repulsive force. The target yaw moment calculation unit 32 ecalculates a target yaw moment based on the lateral direction repulsiveforce, the wheelbase (the distance between the axles), the rear wheelaxle load, and the front wheel axle load. Specifically, a valuemultiplying the ratio of the rear wheel axle load with respect to thevehicle weight (the front wheel axle load+the rear wheel axle load) andthe wheelbase, with respect to the lateral direction repulsive force,shall be the target yaw moment. The target yaw acceleration calculationunit 32 f calculates the target yaw acceleration by multiplying a yawinertia moment coefficient and the target yaw moment. The target yawrate calculation unit 32 g calculates a target yaw rate by multiplyingthe headway time and the target yaw acceleration.

The command turning angle calculation unit 32 h calculates the externaldisturbance suppression command turning angle δ_(st)* by referencing thefollowing formula, based on the target yaw rate φ*, the wheelbaseWHEEL_BASE, the vehicle speed V, and the characteristic velocity of thevehicle V_(ch). Here, the characteristic velocity of the vehicle V_(ch)is a parameter in the well-known “Ackerman Equation,” representing theself-steering characteristics of the vehicle.

δ_(st)*=(φ*×WHEEL_BASE×(1+(V/vCh)²)×180)/(V×M _(—) PI)

where M_PI is a predetermined coefficient.

The limiter processing unit 32 i limits the maximum value and the changerate of the external disturbance suppression command turning angleδ_(st)*. In a conventional steering device (in which the steering unitand the turning unit are mechanically connected), when the steeringangle of the steering wheel 6 in an angle range of the play near theneutral position (for example, 3° to the left and right), the maximumvalue shall be the turning angle range of the front wheels 5FL, 5FRcorresponding to the range of the play (for example, 0.2° to the leftand right).

FIG. 5 is a control block view of the repulsive force calculation unit37 corresponding to the yaw angle. The upper and lower limiter 37 aexecutes an upper and lower limiter operation on the yaw angle. When theyaw angle is a positive value (the yaw angle is positive when the whiteline intersects a line extending in the host vehicle travelingdirection), the upper and lower limiter sets the value to be equal to orgreater than a predetermined value that is capable of suppressingdisturbance, sets the value that is generated by the steering of thedriver to be less than a value when the vehicle will become vibrate (forexample, 1°), and sets the value to 0 when the yaw angle is negative.The yaw angle F/B gain multiplication unit 37 b multiplies a yaw angleF/B gain and the yaw angle after the limiter processing. The yaw angleF/B gain shall be equal to or greater than a predetermined value thatwill avoid shortage in the control amount while securing responsiveness,less than a value when the vehicle will vibrate, and a value at whichthe driver will feel a misalignment in the neutral positions of thesteering angle and the turning angle.

The vehicle speed correction gain multiplication unit 37 c multipliesthe vehicle speed correction gain and the vehicle speed. The vehiclespeed correction gain shall have a characteristic in which the maximumvalue is within the range of 0-70 km/h, gradually decreasing within therange of 70-130 km/h and becoming the minimum value (0) within the rangethat is equal to or greater than 130 km/h. The curvature correction gainmultiplication unit 37 d multiplies the curvature correction gain andthe curvature. The curvature correction gain shall have a characteristicof becoming smaller as the curvature becomes greater, and an upper limitand a lower limit (0) are set thereon. The multiplier 37 e multiplieseach of the outputs from the yaw angle F/B gain multiplication unit 37b, the vehicle speed correction gain multiplication unit 37 c, and thecurvature correction gain multiplication unit 37 d to determine arepulsive force corresponding to the yaw angle.

FIG. 6 is a control block view of a repulsive force calculation unit 38corresponding to the lateral position. The subtracter 38 a determines alateral position deviation by subtracting the distance to the white lineat the forward fixation point from a lateral position threshold valuethat has been set beforehand (for example, 90 cm). The upper and lowerlimiter 38 b executes an upper and lower limiter operation on thelateral position deviation. The upper and lower limiter takes apredetermined positive value when the lateral position deviation is apositive value; this value is 0 when the lateral position deviation is anegative value. The distance correction gain multiplication unit 38 cmultiplies the distance correction gain and the distance to the whiteline at the forward fixation point. The distance correction gain shallhave a characteristic taking the maximum value when the distance to thewhite line is equal to or less than a predetermined value and ofbecoming smaller as the distance becomes longer when exceeding thepredetermined value, and a lower limit is set thereon.

The lateral position F/B gain multiplication unit 38 d multiplies thelateral position F/B gain and the distance to the white line after acorrection has been made by the distance correction gain multiplicationunit 38 c. The lateral position F/B gain shall be equal to or greaterthan a predetermined value that will avoid shortage in the controlamount while securing responsiveness and less than a value when thevehicle will vibrate and a value at which the driver will feel amisalignment in the neutral positions: this is also set to be a valuethat is less than the yaw angle F/B gain of the yaw angle F/B gaincalculation unit 37 b. The vehicle speed correction gain multiplicationunit 38 e multiplies the vehicle speed correction gain and the vehiclespeed. The vehicle speed correction gain shall have the characteristicthat the maximum value is within the range of 0-70 km/h, graduallydecreasing within the range of 70-130 km/h and becoming the minimumvalue (0) within the range of equal to or greater than 130 km/h. Thecurvature correction gain multiplication unit 38 f multiplies thecurvature correction gain and the curvature. The curvature correctiongain shall have a characteristic of becoming smaller as the curvaturebecomes greater, and an upper limit and a lower limit (0) are setthereon. The multiplier 38 g multiplies each of the outputs from thelateral position F/B gain multiplication unit 38 d, the vehicle speedcorrection gain multiplication unit 38 e, and the curvature correctiongain multiplication unit 38 f to determine a repulsive forcecorresponding to the lateral position.

Stability Control Effect

In the first embodiment, a yaw angle F/B control for reducing the yawangle generated by disturbance and a lateral position F/B control forreducing the lateral position change, which is the integrated value ofthe yaw angles generated by the disturbance, are executed as stabilitycontrol. The yaw angle F/B control is executed regardless of the lateralposition when a yaw angle is generated, and the lateral position F/Bcontrol is executed when the distance to the white line becomes equal toor less than a predetermined lateral position threshold value (90 cm).That is, the vicinity of the center of the traveling lane becomes a deadzone for the lateral position F/B control. The control ranges of the twoF/B controls are illustrated in FIG. 7. φ is the yaw angle.

FIG. 8 is a time chart illustrating the yaw angle change when a vehicletraveling on a straight road of a highway receives a sporadic crosswindand when the vehicle is assumed to be traveling in the vicinity of thecenter of the traveling lane. In the yaw angle F/B control, when thevehicle receives a sporadic crosswind and a yaw angle is generated, arepulsive force corresponding to the yaw angle is calculated, a externaldisturbance suppression command turning angle for obtaining therepulsive force is determined, and the SBW command turning angle basedon the steering angle and the vehicle speed is corrected. When a vehicletravels along the traveling lane, especially on a straight road, thedirection of the white line and the host vehicle traveling directionmatch; as a result, the yaw angle will be zero. That is, in the yawangle F/B control of the first embodiment, the yaw angle is assumed tobe generated by disturbance; therefore, enhancing the safety of thevehicle with respect to disturbance, especially traveling straight byreducing the yaw angle is possible, and reducing the corrective steeringamount of the driver is possible.

Conventionally, as those that suppress the effect of disturbance such ascrosswind on the vehicle behavior, that which applies a turning torquefor external disturbance suppression to the steering system is known ina conventional steering device, and that which applies a steeringreaction force component that promotes turning for external disturbancesuppression in known in an SBW system. However, fluctuation in thesteering reaction force is generated in these conventional steeringdevices, imparting discomfort to the driver. In contrast, in thestability control comprising the yaw angle FIB control of the firstembodiment, by focusing attention on the point at which the steeringwheel 6 and the front wheels 5L, 5R can be independently controlled,which is a characteristic of an SBW system in which the steering wheel 6and the front wheels 5L and 5R are mechanically separated, the turningangle of the front wheels 5L, 5R can be controlled based on a commandturning angle that adds the SBW command turning angle, corresponding tothe steering angle and the vehicle speed, and the external disturbancesuppression command turning angle, corresponding to the yaw angle, whilea tire lateral force is inferred based on the steering angle and thevehicle speed; the steering reaction force is controlled based on thecommand steering reaction force corresponding to the inferred tirelateral force and the vehicle speed. That is, since a turning angle forsuppressing disturbance is directly applied to the front wheels 5L, 5R,applying a steering reaction force component that promotes turning forexternal disturbance suppression becomes unnecessary. Furthermore, byapplying a steering reaction force corresponding to the tire lateralforce inferred from the steering angle, fluctuation in the tire lateralforce generated by turning for external disturbance suppression will notbe reflected on the steering reaction force; as a result, the discomfortimparted to the driver can be reduced. In a conventional SBW system, thetire lateral force is inferred from a rack axial force or the turningangle detected by a sensor, and a steering reaction force correspondingto the inferred tire lateral force is applied. Consequently, fluctuationin the tire lateral force generated by turning for external disturbancesuppression will always be reflected in the steering reaction force,creating discomfort for the driver. In the first embodiment, only thetire lateral force that is generated by the steering of the driver isreflected in the steering reaction force, and the steering reactionforce does not fluctuate due to turning for external disturbancesuppression; therefore, the discomfort imparted to the driver can bereduced.

Here, misalignment of the neutral positions of the steering angle andthe turning angle becomes a problem when applying a turning angle forsuppressing disturbance directly onto the front wheels 5L, 5R; however,in the first embodiment, the external disturbance suppression commandturning angle is set to a turning angle range of the front wheels 5FL,5FR (0.2° to the left and right), corresponding to the range of play,when the steering wheel 6 is in the angle range of the play in thevicinity of the steering angle neutral position (3° to the left andright) in a conventional steering device. The generation of a yaw angleby disturbance is more notable when traveling straight than whenturning; when traveling straight, the steering angle is positioned inthe vicinity of the steering angle neutral position. In other words,since correcting the turning angle by the yaw angle F/B control ismostly executed in the vicinity of the steering angle neutral position,suppressing the discomfort that accompanies a neutral misalignment ispossible by suppressing the neutral misalignment amount between thesteering angle and the turning angle, which accompanies the applicationof the external disturbance suppression command turning angle, in therange of the play of the steering. Additionally, since the externaldisturbance suppression command turning angle is limited to the range of0.2° to the left and right, the driver is able to change the travelingdirection of the vehicle in the desired direction by the steering input,even during stability control. That is, since the correction amount ofthe turning angle by the external disturbance suppression commandturning angle is minute with respect to the change amount of the turningangle generated by the steering input of the driver, enhancing thesafety of the vehicle with respect to disturbance is possible withoutinterfering with the steering by the driver.

Conventionally, a lane departure prevention control that applies a yawmoment for preventing the departure of the vehicle, when a travelinglane departure tendency of the vehicle has been detected or a lanemaintenance control that applies a yaw moment to the vehicle so that thevehicle will travel in the vicinity of the center of the traveling laneare known as those that control the lateral movement of the vehicle.However, a lane departure prevention control is a control having acontrol intervention threshold, and the control is not actuated in thevicinity of the center of the traveling lane; therefore, the safety ofthe vehicle with respect to disturbance cannot be secured. Also, since acontrol intervention takes place due to the threshold value even if thedriver wants to pull the vehicle to the edge of the traveling lane, thedriver will experience some difficulty. On the other hand, a lanemaintenance control is a control having a target position (a targetline), so that, while the safety of the vehicle with respect todisturbance can be secured, causing the vehicle to travel a line thatdeviates from the target line is not possible. In addition, since thecontrol will be released when the gripping force of the driver on thesteering wheel is reduced due to a determination that a hands-off stateexists, the driver will have to constantly grip the steering wheel at aforce above a certain level; as a result, there is a large steering loadon the driver. In contrast, the yaw angle F/B control of the firstembodiment does not have a control intervention threshold; therefore,always securing safety with respect to disturbance with a seamlesscontrol is possible. Also, since the above does not have a targetposition, the driver is able to drive the vehicle in a desired line.Additionally, control will not be released even if the steering wheel 6is lightly held, allowing for a reduction in the steering load of thedriver.

FIG. 9 is a time chart illustrating the yaw angle change and the lateralposition change when the lateral position F/B control is not executed,when a vehicle traveling a straight road of a highway receives acontinuous crosswind, and the vehicle is assumed to be traveling in thevicinity of the center of the traveling lane. When a vehicle receives acontinuous crosswind and a yaw angle is generated, the yaw angle will bereduced due to the yaw angle F/B control, but the vehicle will still bereceiving a continuous crosswind and will be drifting. This is becausethe yaw angle F/B control is for reducing the yaw angle and will notcorrect the turning angle when the yaw angle is zero; therefore,directly reducing the lateral position change, which is the integratedvalue of the yaw angles that are generated due to disturbance, is notpossible. Indirectly suppressing the lateral position change(suppressing an increase in the integrated value of the yaw angles) ispossible by making the repulsive force corresponding to the yaw angle alarge value; however, since the maximum value of the externaldisturbance suppression command turning angle is limited to 0.2° to theleft and right so as not to impart discomfort to the driver, effectivelysuppressing the drifting of the vehicle only with yaw angle F/B controlis difficult. Additionally, the yaw angle F/B gain for determining therepulsive force corresponding to the yaw angle is made to be as large avalue as possible because converging the yaw angle before the drivernotices the yaw angle change is necessary; however, since the vehiclewill vibrate if this remains that way, the yaw angle that is multipliedby the yaw angle F/B gain is limited to be equal to or less than theupper limit (1°) by the upper and lower limiter 37 a. In other words,since the repulsive force corresponding to the yaw angle is a repulsiveforce corresponding to a yaw angle that is less than the actual yawangle, this point also demonstrates that effectively suppressing thedrifting of the vehicle only with yaw angle F/B control is difficult.

Therefore, in the stability control of the first embodiment, a lateralposition F/B control is introduced to suppress the vehicle from driftingby a steady disturbance. FIG. 10 is a time chart illustrating the yawangle change and the lateral position change when the lateral positionF/B control has been executed when a vehicle traveling on a straightroad of a highway receives a continuous crosswind, and in the lateralposition F/B control; when a vehicle traveling in the vicinity of thecenter of the traveling lane receives a continuous crosswind and driftsand the distance to the white line becomes equal to or less than alateral position threshold, a repulsive force corresponding to thelateral position change (≈yaw angle integrated value) is calculated. Inthe external disturbance suppression command turning angle calculationunit 32, a external disturbance suppression command turning angle basedon the lateral direction repulsive force, which adds the repulsive forcecorresponding to the lateral position and the repulsive forcecorresponding to the yaw angle, is calculated, and the SBW commandturning angle is corrected. That is, in the lateral position F/Bcontrol, the SBW command turning angle is corrected by a externaldisturbance suppression command turning angle corresponding to thelateral position; as a result, directly reducing the lateral positionchange caused by steady disturbance is possible, and the drifting of thevehicle can be suppressed. In other words, returning the travelingposition of a vehicle conducting a yaw angle F/B control to the vicinityof the center of the traveling lane, which is a dead zone for thelateral position F/B control, is possible.

As described above, the stability control of the first embodimentreduces the yaw angle change due to a transient disturbance with the yawangle F/B control and reduces the yaw angle integrated value (thelateral position change) due to a steady disturbance with the lateralposition F/B control; as a result, the stability control is capable ofenhancing the safety of the vehicle against both transient and steadydisturbances. Furthermore, the stability control of the first embodimentlimits the vehicle behavior that is generated by the control (theapplication of the external disturbance suppression command turningangle) to a level that is not noticed by the driver and a level thatwill not interfere with the vehicle behavior change that is generated bythe steering of the driver; this does not reflect the change in theself-aligning torque generated by the control on the steering reactionforce and, thus, can be executed without the driver being aware thatstability control is taking place. As a result, simulating the behaviorof a vehicle in order to have a vehicle body specification withexcellent stability against disturbance is possible. The lateralposition F/B gain for determining the repulsive force corresponding tothe lateral position in the lateral position F/B control is set to avalue that is less than the yaw angle F/B gain. As described above, theyaw angle F/B control must be responsive due to the necessity ofconverging the yaw angle before the driver feels a change in the yawangle caused by a transient disturbance, whereas the lateral positionF/B control does not require as much responsiveness as the yaw angle F/Bcontrol, because stopping the increase in the lateral position change isrequired and the lateral position takes time to change due to theaccumulation of the yaw angle integrated value. In addition, this isbecause, if the lateral position F/B gain were to be increased, thecontrol amount will change greatly according to the magnitude of thedisturbance, and discomfort will be imparted on the driver.

Lateral Force Offset Unit

FIG. 11 is a control block view of a lateral force offset unit 34. Acurvature calculation unit 34 a calculates the curvature of the whiteline at the forward fixation point. Either the left or the right whiteline can be used as the white line. An upper and lower limiter 34 bexecutes an upper and lower limiter operation on the vehicle speed. ASAT gain calculating unit 34 c calculates an SAT gain corresponding tothe vehicle speed based on the vehicle speed after the limiteroperation. The SAT gains shall have a characteristic of becoming alarger gain as the vehicle speed increases, and an upper limit is setthereon. The multiplier 34 d determines the lateral force offset amountby multiplying the curvature and the SAT gain. A limiter processing unit34 e limits the maximum value and the upper limit of the change rate ofthe lateral force offset amount. For example, the maximum value is 1,000N, and the upper limit of the change rate is 600 N/s.

Effect of the Reaction Force Offset Control Corresponding to theCurvature

The reaction force offset control corresponding to the curvaturedetermines a larger lateral force offset amount as the curvature of thewhite line increases, which is subtracted from the tire lateral force.The steering reaction force torque corresponding to the tire lateralforce that is calculated by the SAT calculation unit 35, that is, thesteering reaction force characteristic representing the steeringreaction force torque corresponding to the self-aligning torque isthereby offset in the same coding direction as the self-aligning torqueas the curvature of the white line increases, as illustrated in FIG. 12.FIG. 12 illustrates a case of a right curve, and in the case of a leftcurve, the offset is in the opposite direction of that depicted in FIG.12.

Conventionally, in an SBW system in which the steering unit and theturning unit are mechanically separated, a steering reaction forcecharacteristic that simulates a steering reaction force corresponding tothe self-aligning torque in a conventional steering device is set, andthe steering reaction force is applied to the steering wheel based onthe steering reaction force characteristic; at this time, therelationship between the steering angle of the steering wheel and theturning torque of the driver has the characteristic A illustrated inFIG. 13. That is, the absolute value of the turning torque increases asthe absolute value of the steering angle increases, and the changeamount of the turning torque with respect to the change amount of thesteering angle increases when the absolute value of the steering angleis small, as compared to when large.

Here, a case is considered in which the driver changes the steeringretention torque to adjust the course during turning. In FIG. 13, whenthe driver reduces the steering retention torque to T₂ from a state inwhich the steering angle θ₁ is retained with a steering retention torqueT₁, the steering angle becomes θ₂, and the turning angle of the frontwheels 5L, 5R decreases due to the decrease in the steering angle. Atthis time, due to the steering reaction force characteristic in the SBWsystem described above, the steering angle varies greater with respectto the change in the steering reaction force torque as the curvature ofthe curve increases. In other words, the sensitivity of the vehicle withrespect to the steering torque increases as the curvature of the curveincreases; as a result, there is a problem that adjusting the course isdifficult.

In contrast, in the reaction force offset control corresponding to thecurvature of the first embodiment, by offsetting the steering reactionforce characteristic more, which represents the steering reaction forcetorque corresponding to the self-aligning torque, in the same directionas the self-aligning torque, the characteristic representing therelationship between the steering angle and the turning torque is offsetin the same coding direction as the steering angle, as illustrated inFIG. 14, and characteristic A changes to characteristic B. The changeamount of the steering angle with respect to the change amount of thesteering retention torque thereby decreases as the curvature of thewhite line increases; even when the driver reduces the steeringretention torque to T4 and when the reduction amount of the steeringretention torque ΔT₃₋₄ is the same as the reduction amount of the priorart ΔT₁₋₂, the reduction amount of the steering angle Δθ₁₋₄ will becomesmaller than the reduction amount of the prior art Δ₁₋₂. That is,variation in the steering angle with respect to the change in thesteering retention torque can be made smaller as the curvature of thecurve increases, and the sensitivity of the vehicle with respect to thesteering torque can be reduced; as a result, behavior change in thevehicle becomes gradual, and facilitating the adjustment of the courseby the driver is possible. Additionally, since the steering retentiontorque T₃ (<T₁) for maintaining the steering angle θ₁ can be madesmaller than that of the prior art, the steering load of the driverwhile turning can be reduced.

Conventionally, technology that aims to reduce the steering load of thedriver while turning, which reduces the slope of the steering reactionforce characteristic more as the curvature of the white line increases,is known; however, in the conventional technology, variability in thesteering angle with respect to the change in the steering retentiontorque increases as the curvature increases, so the sensitivity of thevehicle with respect to the steering torque increases. That is, inrealizing a reduction in the steering load of the driver while turningand facilitating the adjustment of the course are possible by offsettingthe steering reaction force characteristic in the same direction as theself-aligning torque in accordance with the curvature of the white line.

Steering Reaction Force Torque Offset Unit

FIG. 15 is a control block view of a steering reaction force torqueoffset unit 36. A yaw angle calculation unit 36 a calculates the yawangle at the forward fixation point. The yaw angle can be easily andprecisely detected by calculating the yaw angle based on an image of thetraveling path captured by the camera 17. Either the left or the rightwhite line can be used as the white line. A lateral position calculationunit 36 b calculates each of the lateral position with respect to theleft and right white lines at the forward fixation point and the lateralposition with respect to the left and right white lines at the currentposition. Here, when the host vehicle moves to an adjacent travelinglane beyond the white line, that is, when a lane change occurs, thelateral position calculation unit 36 b replaces the lateral positionwith respect to the left and right white lines at the current position.That is, the lateral position with respect to the left white line beforereaching the white line is set as the lateral position with respect tothe right white line after reaching the white line; the lateral positionwith respect to the right white line before reaching the white line isset as the lateral position with respect to the left white line afterreaching the white line. When lane changing to a traveling lane with adifferent lane width, the lateral position is corrected by multiplyingthe value W₂/W₁, obtained by dividing the lane width W₂ of the travelinglane after the lane change by the lane width W₁ of the traveling lanebefore the lane change, by the replaced lateral position. Here, the lanewidth information of each traveling lane is acquired from a navigationsystem 24. A reaction force calculation unit 39 corresponding to thedeviation margin time calculates a reaction force corresponding to thedeviation margin time based on the vehicle speed and the lateralposition with respect to the left and right white lines at the forwardfixation point. The details of the reaction force calculation unit 39corresponding to the deviation margin time will be described below. Areaction force calculation unit 40 corresponding to the lateral positioncalculates the reaction force corresponding to the lateral position,based on the lateral position with respect to the left and right whitelines at the current position. The details of the reaction forcecalculation unit 40 corresponding to the lateral position will bedescribed below. A reaction force selection unit 36 c selects that withthe larger absolute value among the reaction force corresponding to thedeviation margin time and the reaction force corresponding to thelateral position as the steering reaction force torque offset amount. Alimiter processing unit 36 d limits the maximum value and the upperlimit of the change rate of the steering reaction force torque offsetamount. For example, the maximum value is 2 Nm, and the upper limit ofthe change rate is 10 Nm/s.

FIG. 16 is a control block view of a reaction force calculation unit 39corresponding to the deviation margin time. A multiplier 39 a determinesthe lateral speed of the vehicle by multiplying the vehicle speed andthe yaw angle. A divider 39 b determines the deviation margin time withrespect to the left white line by dividing the lateral position withrespect to the left white line at the forward fixation point by thelateral speed. A divider 39 c determines the deviation margin time withrespect to the right white line by dividing the lateral position withrespect to the right white line at the forward fixation point by thelateral speed. A deviation margin time selection unit 39 d selects theshorter of the deviation margin times with respect to the left and rightwhite lines as the deviation margin time. A reaction force calculationunit 39 e corresponding to the deviation margin time calculates thereaction force corresponding to the deviation margin time, based on thedeviation margin time. The reaction force corresponding to the deviationmargin time is inversely proportional to the deviation margin time(proportional to the inverse of the deviation margin time) and has thecharacteristic of becoming almost zero at three seconds or more.

FIG. 17 is a control block view of a reaction force calculation unit 40corresponding to the lateral position. The subtracter 40 a determinesthe lateral position deviation with respect to the left lane bysubtracting the lateral position with respect to the left lane from atarget left lateral position that is set beforehand (for example, 90cm). A subtracter 40 b determines the lateral position deviation withrespect to the right lane by subtracting the lateral position withrespect to the right lane from a target right lateral position that isset beforehand (for example, 90 cm). A lateral position deviationselection unit 40 c selects the larger of the lateral positiondeviations with respect to the left and right lanes as the lateralposition deviation. A reaction force calculation unit 40 d correspondingto the lateral position deviation calculates the reaction forcecorresponding to the lateral position, based on the lateral positiondeviation. The reaction force corresponding to the lateral position isset to have a characteristic of increasing as the lateral positiondeviation increases, and an upper limit is set thereon.

Effect of the Reaction Force Offset Control Corresponding to the LateralPosition

The reaction force offset control corresponding to the lateral positionadds the reaction force corresponding to the lateral position to thesteering reaction force torque to determine the steering reaction forcetorque offset amount. The steering reaction force characteristicrepresenting the steering reaction force torque corresponding to theself-aligning torque is thereby offset in a direction in which theabsolute value of the steering reaction force torque increases as thedistance to the white line decreases, as illustrated in FIG. 18. FIG. 18illustrates a case of being close to the right lane, and, in the case ofbeing close to the left lane, the offset is in the opposite direction ofFIG. 18.

Here, a case is considered in which the traveling position of thevehicle shifts to the right side due to the driver suddenly steering tothe right, after which the driver returns the traveling position to thevicinity of the center of the traveling lane with corrective steering,in a conventional steering reaction force control. The steering angleand the steering torque when the driver conducts a sudden operationshall the position of point P₁ on the characteristic A in FIG. 19. Thecharacteristic A shall be a characteristic representing the relationshipbetween the steering angle and the steering torque when setting asteering reaction force characteristic simulating a conventionalsteering device in the same manner as FIG. 13. Since turning the frontwheel is necessary in order to return the traveling position to thevicinity of the center of the traveling lane from this state, followingthe increased steering to the steering angle neutral position, thedriver increases the steering from the steering angle neutral positionto match the steering wheel to a target angle θ₅. At this time, in theconventional technology described above, the steering angle neutralposition (the steering angle zero point) and the steering torque neutralposition (the steering torque zero point) match, and decreasing thesteering torque until the steering angle is in the neutral positionwhile increasing the steering torque after exceeding the steering angleneutral position is necessary. In other words, when conductingcorrective steering straddling the steering angle neutral position, thesign of the steering torque is inverted, and the direction in which thedriver controls the force is switched; since the change amount of thesteering angle with respect to the change amount of the steering torqueis significantly smaller in the vicinity of the steering torque neutralposition, as compared to other steering angle regions, the steering loadon the driver is large, and controlling the steering wheel to be at thetarget angle θ₅ is difficult. Thus, there is the problem that thetraveling position of the vehicle is more readily overshot, leading toan increase in the corrective steering amount.

In contrast, in the reaction force offset control corresponding to thelateral position of the first embodiment, by offsetting the steeringreaction force torque corresponding to the self-aligning torque more ina direction in which the absolute value of the steering reaction forcetorque increases as the distance to the white line decreases, thecharacteristic representing the relationship between the steering angleand the turning torque is offset in the direction in which the absolutevalue of the steering torque increases, as illustrated in FIG. 20, andcharacteristic A changes continuously to characteristic C, as thedistance to the white line decreases. At this time, increasing thesteering torque is necessary in order to maintain the steering angle;therefore, if the steering torque is constant, the steering wheel 6gradually returns to the steering angle neutral position (pointP₁->point P₂), thereby suppressing the traveling position of the vehiclefrom shifting to the right side due to a sudden increase in steering bythe driver. On the other hand, when the driver maintains the steeringangle, the steering angle and the steering torque move from point P₁ topoint P₃. When the driver conducts corrective steering from this state,since the steering torque neutral position is offset more to thesteering increase side than the steering angle neutral position incharacteristic C, the sign of the steering torque is not inverted beforereaching the steering torque neutral position when the steeringincreases from the steering angle neutral position. Thus, the driver isable to control the turning angle of the front wheels 5L, 5R by onlyreducing the steering torque and stopping the rotation of the steeringwheel 6 when the steering wheel 6 is turned to the target angle. Thatis, the reaction force offset control corresponding to the lateralposition of the first embodiment is able to facilitate the correctivesteering from the driver since the direction in which the drivercontrols the force is not readily switched. As a result, the travelingposition of the vehicle is not readily overshot, and the correctivesteering amount can be reduced.

Conventionally, a technology is known in which the object is to preventthe traveling position from shifting due to the driver suddenlyincreasing the steering reaction force when approaching the white line;however, in the conventional technology, the steering wheel is simplymade to be heavier when approaching the white line, and the steeringtorque neutral position in the steering reaction force characteristicalways matches with the steering angle neutral position; therefore, thesign of the steering torque is inverted in corrective steering thatstraddles the steering angle neutral position, and the steering load ofthe driver is not reduced. In other words, by offsetting the steeringreaction force torque corresponding to the self-aligning torque more ina direction in which the absolute value of the steering reaction forcetorque increases as the distance to the white line decreases, realizingboth the suppression of the shifting of the traveling position and areduction in the steering load of the driver is possible.

Additionally, in the reaction force offset control corresponding to thelateral position of the first embodiment, the offset amount isconfigured to be greater as the distance to the white line decreases; asa result, the steering torque neutral position is offset to a positionthat is further separated from the steering angle neutral position asthe distance to the white line decreases. When the driver conductscorrective steering to return the traveling position of the vehicle tothe vicinity of the center of the traveling lane, increasing thesteering increase amount from the steering angle neutral position as tothe white line comes closer is necessary. At this time, when the offsetamount of the steering torque neutral position with respect to thesteering angle neutral position is small, there is the possibility thatthe steering torque surpasses the neutral position and the sign of thesteering torque is inverted before the steering wheel turns to thetarget angle. Thus, suppressing the steering torque from surpassing theneutral position is possible by increasing the offset amount as thedistance to the white line decreases.

In the reaction force offset control corresponding to the lateralposition of the first embodiment, the lateral position calculation unit36 b switches the lateral position with respect to the left and rightwhite lines at the current position when the host vehicle reaches thewhite line. The reaction force offset control corresponding to thelateral position is configured so that the host vehicle readily returnsto the vicinity of the center of the traveling lane by increasing thesteering reaction force as the host vehicle gets farther away from thevicinity of the center of the traveling lane. In other words, the yawangle integrated value (the lateral position change) is considered to bea disturbance, and the steering reaction force is controlled so that thevehicle is guided in a direction in which the yaw angle integrated valueis eliminated. Consequently, resetting the yaw angle integrated valuewhen a lane change has been conducted is necessary. This is because, ifthe yaw angle integrated value is not reset, the steering reaction forcefor returning the vehicle to the vicinity of the center of the travelinglane before the lane change will continue to act even after the lanechange, and the operation of the driver will be inhibited. If theintegrated value is simply set to be zero, guiding the vehicle to thevicinity of the center of the traveling lane after the lane change willnot be possible.

Therefore, in the first embodiment, since a deliberate operation of thedriver can be considered when the host vehicle reaches the white line,in that case, by switching the lateral position with respect to the leftand right white lines at the current position, in other words, byinverting the sign of the yaw angle integrated value, the position towhich the host vehicle is guided is switched from the vicinity of thecenter of the traveling lane before the lane change to the vicinity ofthe center of the traveling lane after the lane change; therefore, asteering reaction force for guiding the host vehicle to the vicinity ofthe center of the traveling lane after the lane change can be generated.At this time, in order to consider the ratio W₂/W₁ of the lane width W₂of the traveling lane after the lane change with respect to the lanewidth W₁ of the traveling lane before the lane change, setting anaccurate lateral position is possible, and setting an optimum offsetamount for guiding the host vehicle to the vicinity of the center of thetraveling lane is possible.

Effect of the Reaction Force Offset Control Corresponding to theDeviation Margin Time

The reaction force offset control corresponding to the deviation margintime adds the reaction force corresponding to the deviation margin timeto the steering reaction force torque to determine the steering reactionforce torque offset amount. The steering reaction force characteristicrepresenting the steering reaction force torque corresponding to theself-aligning torque is thereby offset in a direction in which theabsolute value of the steering reaction force torque increases asdeviation margin time decreases, as illustrated in FIG. 18. FIG. 18illustrates a case of being close to the right lane, and in the case ofbeing close to the left lane, the offset is in the opposite direction ofthat shown in FIG. 18.

Accordingly, the characteristic representing the relationship betweenthe steering angle and the steering torque is offset in a direction inwhich the absolute value of the steering torque increases, andcharacteristic A changes continuously to characteristic C, as thedeviation margin time decreases, as illustrated in FIG. 20. At thistime, increasing the steering torque in order to maintain the steeringangle is necessary; therefore, if the steering torque is constant, thesteering wheel 6 gradually returns to the steering angle neutralposition (point P₁->point P₂), thereby suppressing the travelingposition of the vehicle from shifting to the right side due to thedriver suddenly steering. On the other hand, when the driver maintainsthe steering angle, the steering angle and the steering torque movesfrom point P₁ to point P₃. When the driver conducts corrective steeringfrom this state, since the steering torque neutral position is offsetmore to the steering increase side than the steering angle neutralposition in characteristic C, the sign of the steering torque is notinverted before reaching the steering torque neutral position whensteering increases from the steering angle neutral position. Thus, thedriver is able to control the turning angle of the front wheels 5L, 5Rby only reducing the steering torque and stopping the rotation of thesteering wheel 6 when the steering wheel 6 is turned to the targetangle. That is, the reaction force offset control corresponding to thedeviation margin time of the first embodiment is able to facilitate thecorrective steering of the driver since the direction in which thedriver controls the force is not readily switched. As a result, thetraveling position of the vehicle is not readily overshot, and thecorrective steering amount can be reduced.

Additionally, in the reaction force offset control corresponding to thedeviation margin time of the first embodiment, the offset amount isconfigured to increase as the deviation margin time decreases; as aresult, the steering torque neutral position is offset to a positionthat is further separated from the steering angle neutral position asthe deviation margin time decreases. When the driver conducts correctivesteering for returning the traveling position of the vehicle to thevicinity of the center of the traveling lane, the vehicle is more likelyto be closer to the white line as the deviation margin time decreases,and increasing the amount of steering from the steering angle neutralposition as the white line becomes closer is necessary. At this time,when the offset amount of the steering torque neutral position withrespect to the steering angle neutral position is small, there is thepossibility that the steering torque surpasses the neutral position andthe sign of the steering torque is inverted before the steering wheel isturned to the target angle. Thus, suppressing the steering torque fromsurpassing the neutral position is possible by increasing the offsetamount as the distance to the white line decreases.

Effect of the Reaction Force Offset Control Corresponding to the LateralPosition and the Deviation Margin Time

In the steering reaction force control unit 20, that with the largerabsolute value from among the reaction force corresponding to thedeviation margin time and the reaction force corresponding to thelateral position is selected as the steering reaction force torqueoffset amount in the steering torque offset unit 36, and the steeringreaction force torque offset amount is added to the steering reactionforce torque in the adder 20 c. The steering reaction forcecharacteristic is thereby offset in a direction in which the absolutevalue of the steering reaction force torque increases in accordance withthe deviation margin time or the lateral position. In the reaction forceoffset control corresponding to the deviation margin time, the reactionforce corresponding to the deviation margin time is zero when the hostvehicle and the white line are parallel, that is, when the yaw angle iszero. Consequently, even if the host vehicle is in a position close tothe white line, when the yaw angle is small, only a small reaction forcecan be output. In contrast, in the reaction force offset controlcorresponding to the lateral position, a reaction force (a reactionforce corresponding to the lateral position) is generated proportionateto the distance to the white line; therefore, a larger reaction forcecan be output as the distance to the white line decreases, and readilyreturning the host vehicle to the vicinity of the center of thetraveling lane is possible.

On the other hand, in the reaction force offset control corresponding tothe lateral position, when the host vehicle is in the vicinity of thecenter of the traveling lane, the reaction force corresponding to thelateral position is zero. Consequently, even in the vicinity of thecenter of the traveling lane, when the yaw angle is large and thevehicle speed is high, the white line is reached in a short period oftime while increasing the steering reaction force with goodresponsiveness is difficult. In contrast, in the reaction force offsetcontrol corresponding to the deviation margin time, since a reactionforce (a reaction force corresponding to the deviation margin time) isgenerated in accordance with the deviation margin time, and the reactionforce has the characteristic of rapidly increasing when the deviationmargin time becomes equal to or less than 3 seconds, suppressing lanedeparture by increasing the steering reaction force is possible evenwhen reaching the white line in a short period of time. Thus, bycombining the reaction force offset control corresponding to thedeviation margin time and the reaction force offset controlcorresponding to the lateral position, effectively suppressing lanedeparture while applying a stable reaction force in accordance with thedistance to the white line is possible. At this time, by using that withthe larger absolute value from among the reaction force corresponding tothe deviation margin time and the reaction force corresponding to thelateral position, always applying the optimum required steering reactionforce is possible.

Reaction Force Offset Control Suppression Effect when the White Line onOne Side is Lost

In a system that continuously assists steering during traveling usingthe information from left and right white lines of the traveling lane,such as the reaction force offset control corresponding to the lateralposition and the deviation margin time of the first embodiment, when theloss of the white line occurs, in which information on the white linethat is farther from the host vehicle among the left and right whitelines cannot be obtained, interrupting control is necessary. While amethod of inferring the position of the white line from the past lanewidth to continue control can be conceived as an alternative method, ona road surface typified by branching/merging roads, a misalignmentoccurs between the actual lane width and the lane width used forinference; as a result, accurately inferring the position of the lostwhite line is difficult, making this an unfavorable method. In thereaction force offset control suppression operation of the firstembodiment, when a state in which the white line that is farther fromthe host vehicle is lost has continued for a predetermined time (countvalue=threshold value) and the steering reaction force torque offsetamount is below the previous value, the reaction force offset controlsuppression operation is started, and an increase in the steeringreaction force torque offset amount is inhibited while a decrease ispermitted. Since the steering reaction force torque offset amountbecomes a larger value as the distance to the white line decreases, whenthe driver is steering in a direction that brings the vehicle closer tothe white line, the steering reaction force torque offset amountincreases, which also increases the steering reaction force. At thistime, the driver is steering while assuming an increase in the steeringreaction force that is generated by the reaction force offset controlcorresponding to the lateral position and the deviation margin time;therefore, if the increase in the steering reaction force torque offsetamount is inhibited at this time, the driver will experience anunexpected steering reaction force, imparting discomfort. On the otherhand, when the steering of the driver to bring the vehicle closer to thewhite line has been completed, the vehicle will head toward the centerof the lane, so that the steering reaction force torque offset amountwill begin to decrease. At this time, the driver is steering whileassuming a decrease in the steering reaction force that is generated bythe reaction force offset control corresponding to the lateral positionand the deviation margin time; therefore, even if an increase in thesteering reaction force torque offset amount is inhibited at this time,the driver will experience an expected steering reaction force, reducingthe discomfort that is imparted to the driver.

The effects listed below can be obtained with the first embodiment, asdescribed above.

(1) The present invention comprises an image processing unit 21 (a whiteline detection means) for detecting a left or a right white line of atraveling lane; a steering reaction force torque offset unit 36 (a whiteline approach suppression steering reaction force calculation means)that, when a host vehicle approaches one of the detected left and rightwhite lines, calculates a steering reaction force torque offset amount(a white line approach suppression steering reaction force) forsuppressing steering in a direction in which the host vehicle approachesthe white line and makes the steering reaction force torque offsetamount greater as the distance between the host vehicle and the whiteline decreases; a steering reaction force control unit 20 (a steeringreaction force control means) for controlling a steering reaction forcethat is applied to a steering unit 1 that receives steering input fromthe driver based on the steering reaction force torque offset amount;and a reaction force offset control suppression unit 43 (a controlmeans) for limiting the steering reaction force torque offset amount ifa determination is made that the other among the left and right whitelines cannot be detected and an increase gradient of the steeringreaction force torque offset amount becomes equal to or less than apredetermined increase gradient. By initiating a limit on the steeringreaction force torque offset amount at a point in time when theincreasing tendency of the steering reaction force torque offset amountends, suppressing the steering reaction force from deviating from theexpectation of the driver is possible, and reducing the discomfortimparted to the driver is possible.

(2) The reaction force offset control suppression unit 43 limits thesteering reaction force torque offset amount when the steering reactionforce torque offset amount is below the previous value. Since thevehicle behavior becomes closer to the vehicle behavior expected by thedriver by initiating a limit on the steering reaction force torqueoffset amount at a point in time when the steering reaction force torqueoffset amount displays a decreasing tendency, discomfort imparted to thedriver can be further reduced.

(3) The reaction force offset control suppression unit 43 inhibits anincrease in the steering reaction force torque offset amount and permitsa decrease. The vehicle behavior thereby becomes closer to the vehiclebehavior expected by the driver, as compared to a case in which thesteering reaction force torque offset amount is maintained, and furtherreducing the discomfort imparted to the driver is possible.

(4) The reaction force offset control suppression unit 43 determinesthat the other among the left and right white lines is undetectable whena state in which the other among the left and right white lines isundetectable has continued for a predetermined time. Since the reactionforce offset control corresponding to the lateral position and thedeviation margin time can thereby be continued when the loss of thewhite line occurs for a relatively short period of time, such as whenthe white line becomes detectable immediately after becomingundetectable, suppressing a lane departure is possible.

(5) The steering reaction force control unit 20 applies a steeringreaction force to the steering unit 1 based on a command steeringreaction force obtained by adding the steering reaction force torqueoffset amount to the steering reaction force torque that is obtained bya predetermined steering reaction force characteristic, corresponding toa self-aligning torque that acts on the left and right front wheels 5FL,5FR. Since suppressing the sign of the steering torque from beinginverted when a driver performs corrective steering that straddles thesteering angle neutral position is thereby possible, facilitating theadjustment of the course by the driver in addition to suppressing lanedeparture is possible.

(6) The control unit 1 comprises a curvature calculation unit 34 a (acurvature calculation means) that is mechanically separated from aturning unit 2, which turns the left and right front wheels 5FL, 5FR anddetects the curvature of a white line; a lateral force offset unit 34(an offset amount calculation means) for calculating a lateral forceoffset amount that is greater as the detected curvature increases,wherein the steering reaction force control unit 20 applies a steeringreaction force to the steering unit 1 based on a command steeringreaction force obtained from a self-aligning torque after being offset,which is obtained by subtracting the lateral force offset amount fromthe self-aligning torque that acts on the left and right front wheels5FL, 5FR and a predetermined steering reaction force characteristiccorresponding to the self-aligning torque; the steering reaction forcetorque offset unit 36 does not limit the lateral force offset amounteven when a determination is made that the other among the left andright white lines is undetectable. Since the curvature can be detectedbased on either the left or the right white line, the reaction forceoffset control corresponding to the curvature is not at all affectedeven if the other among the left and right white lines becomesundetectable. That is, realizing both a reduction in the steering loadof the driver while turning and facilitating the adjustment of thecourse are possible by continuing the reaction force offset controlcorresponding to the curvature.

(7) When a host vehicle approaches either a left or a right white line,a steering reaction force torque offset amount that increases as thedistance between the host vehicle and the white line decreases iscalculated; when applying a steering reaction force to the steering unit1 that receives steering input from the driver based on the steeringreaction force torque offset amount, if a determination is made that theother among the left and right white lines cannot be detected and anincrease gradient of the steering reaction force torque offset amountbecomes equal to or less than a predetermined increase gradient, thesteering reaction force torque offset amount is limited. By initiating alimit on the steering reaction force torque offset amount at a point intime when the increasing tendency of the steering reaction force torqueoffset amount ends, suppressing the steering reaction force fromdeviating from the expectation of the driver is possible, and thediscomfort imparted to the driver can be reduced.

(8) The present invention comprises an image processing unit 21 (asensor) for detecting a left or a right white line of a traveling laneand a steering torque offset amount 20 (a controller) in which, when ahost vehicle approaches either the left or right white line, a steeringreaction force torque offset amount that increases as the distancebetween the host vehicle and the white line decreases is calculated;when applying a steering reaction force to a steering unit 1 thatreceives steering input from the driver based on the steering reactionforce torque offset amount, if a determination is made that the otheramong the left and right white lines cannot be detected and an increasegradient of the steering reaction force torque offset amount becomesequal to or less than a predetermined increase gradient, the steeringreaction force torque offset amount is limited. By initiating a limit onthe steering reaction force torque offset amount at a point in time whenthe increasing tendency of the steering reaction force torque offsetamount ends, suppressing the steering reaction force from deviating fromthe expectation of the driver is possible, and the discomfort impartedto the driver can be reduced.

Embodiment 2

FIG. 21 is a system view illustrating a steering system of a vehicle ofthe second embodiment. The portions common with the first embodimenthave been given the same names and codes, and their explanations havebeen omitted. The steering device of the second embodiment is mainlyconfigured by a steering unit 1, a turning unit 2, and an EPS controller25, and the steering unit 1 that receives steering input from a driverand the turning unit 2 that turns the left and right wheels (theturnable wheels) 5FL, 5FR are mechanically coupled. The steering unit 1comprises a steering wheel 6, a column shaft 7, and a torque sensor 26.The torque sensor 26 detects the steering torque of the driver that isinput from the steering wheel 6 to the column shaft 7. The turning unit2 comprises a pinion shaft 11, a steering gear 12, and a power steeringmotor 27. The pinion shaft 11 is connected to the column shaft 7 via atorsion bar of the torque sensor 26. The power steering motor 27 is, forexample, a brushless motor, in which the output shaft is connected to arack gear 15 via an unillustrated decelerator, and outputs an assisttorque for assisting the steering force of the driver to a rack 16 inresponse to a command from the EPS controller 25.

The vehicle speed (the vehicle body speed) detected by an image of thetraveling road in front of the host vehicle captured by a camera 17 anda vehicle speed sensor 18, in addition to the above-described torquesensor 26, are input into the EPS controller 25. The EPS controller 25comprises an assist torque control unit (an assist torque control means,a controller) 28 and an image processing unit 21. The assist torquecontrol unit 28 generates a command assist torque, based on each pieceof input information, and outputs the generated command assist torque toan electric current driver 29. The electric current driver 29 controls acommand electric current to the power steering motor 27 with torquefeedback that matches an actual assist torque that is inferred from acurrent value of the power steering motor 27 with the command assisttorque. The image processing unit 21 recognizes the left and right whitelines of a traveling lane (the traveling path dividing lines) by imageprocessing, such as by edge extraction from an image of a traveling pathin front of a host vehicle captured by a camera 17.

Assist Torque Control Unit

FIG. 22 is a control block view of an assist torque control unit 28. Theassist torque calculation unit 41 calculates an assist torque withreference to an assist torque map that is set beforehand, based on thesteering torque and the vehicle speed. The assist torque characteristicin the assist torque map has a characteristic of increasing as theabsolute value of the steering torque increases or as the vehicle speeddecreases. The assist torque offset unit 42 calculates an assist torqueoffset amount for offsetting the assist torque characteristic in anassist torque offset control corresponding to the lateral position orthe deviation margin time, based on the vehicle speed and an image of atraveling path in front of the host vehicle. The details of the assisttorque offset unit 42 will be described below. A subtracter 28 a outputsa value obtained by subtracting the assist torque offset amount from theassist torque to the electric current driver 29 as the final commandassist torque.

Assist Torque Offset Unit

FIG. 23 is a control block view of an assist torque offset unit 42. Areaction force selection unit 42 c selects that with the larger absolutevalue from among the reaction force corresponding to the deviationmargin time and the reaction force corresponding to the lateral positionas the assist torque offset amount. An assist torque offset controlsuppression unit 44 comprises a counter 44 a and an assist torque offsetamount limiting unit 44 b. When the white line farther from the hostvehicle from among the left and right white lines becomes undetectable,the counter 44 a starts to count up; when the count value reaches athreshold value, the count value is reset (=0), and an assist torqueoffset control suppression flag is set (=1). The assist torque offsetcontrol suppression flag is reset (=0) when the white line becomesundetectable. The assist torque offset amount limiting unit 44 b startsan assist torque offset control suppression operation when an assisttorque offset control suppression flag is set and the assist torqueoffset amount is below the previous value. In the assist torque offsetcontrol suppression operation, the assist torque offset amount islimited. Specifically, when the assist torque offset amount exceeds theprevious value, the previous value is output as the assist torque offsetamount, and the current value is output when equal to or greater thanthe previous value.

Assist torque offset control effect when the white line on one side islost

In the assist torque offset control suppression process of the secondembodiment, when a state in which the white line that is farther fromthe host vehicle is lost has continued for a predetermined time (countvalue=threshold value) and the assist torque offset amount is below theprevious value, the assist torque offset control suppression operationis started, and an increase in the assist torque offset amount isinhibited while a decrease is permitted. Since the assist torque offsetamount becomes a larger value as the distance to the white linedecreases, when the driver is steering in a direction that brings thevehicle closer to the white line, the assist torque offset amountincreases, which also increases the steering reaction force. At thistime, the driver is steering while assuming an increase in the steeringreaction force that is generated by the assist torque offset controlcorresponding to the lateral position and the deviation margin time; asa result, if the increase in the assist torque offset amount isinhibited at this time, the result will be an unexpected steeringreaction force for the driver, imparting discomfort. On the other hand,when the steering of the driver to bring the vehicle closer to the whiteline has been completed, the vehicle will head toward the center of thelane, so that the assist torque offset amount will begin to decrease. Atthis time, the driver is steering while assuming a decrease in thesteering reaction force that is generated by the assist torque offsetcontrol corresponding to the lateral position and the deviation margintime; as a result, even if an increase in the assist torque offsetamount is inhibited at this time, the result will be an expectedsteering reaction force by the driver, reducing the discomfort that isimparted to the driver.

1. A steering control device comprising: a white line detection unitprogrammed to a left or a right white line of a traveling lane; a whiteline approach suppression steering reaction force calculating unitprogrammed to calculate a white line approach suppression steeringreaction force that suppresses steering in a direction in which a hostvehicle approaches the left or right white line, when a host vehicleapproaches either the left or the right white line, a steering reactionforce control unit programmed to control the steering reaction forceapplied to a steering unit that receives steering input from a driver,based on the white line approach suppression steering reaction force;and a limiting unit programmed to limit the white line approachsuppression steering reaction force, upon determining that the otheramong the left and right white lines is undetectable and a distancebetween the host vehicle and the left or right white line is decreasing.2. The steering control device according to claim 1, wherein thelimiting unit is further programmed to limit the white line approachsuppression steering reaction force, when the white line approachsuppression steering reaction force is below a previous value.
 3. Thesteering control device according to claim 1, wherein the limiting unitis further programmed to inhibit an increase in the white line approachsuppression steering reaction force while permitting a decrease in thewhite line approach suppression steering reaction force.
 4. The steeringcontrol device according to claim 1, wherein the limiting unit isfurther programmed to determine that the other among the left and rightwhite lines is undetectable, when a state in which the other among theleft and right white lines is undetectable has continued for apredetermined time.
 5. The steering control device according to claim 1,wherein the steering reaction force control unit is further programmedto apply a steering reaction force to the steering unit based on acommand steering reaction force obtained by adding the white lineapproach suppression steering reaction force to the steering reactionforce that is obtained from a predetermined steering reaction forcecharacteristic, corresponding to a self-aligning torque that acts on aturnable wheel.
 6. The steering control device according to claim 1,wherein the steering unit is mechanically separated from a turning unitfor turning a turnable wheel, the steering unit comprising: a curvaturedetection unit programmed to detect a curvature of a white line; and anoffset amount calculation unit programmed to calculate an offset amountthat is greater as the detected curvature increases; the steeringreaction force control unit is further programmed to apply a steeringreaction force to the steering unit based on a command steering reactionforce obtained from a self-aligning torque after being offset, which isobtained by subtracting the offset amount from the self-aligning torquethat acts on the turnable wheel and a predetermined steering reactionforce characteristic corresponding to the self-aligning torque, and thelimiting unit being further programmed not to limit the offset amount,even when a determination has been made that the other among the leftand right white lines is undetectable.
 7. A steering control devicecomprising: a controller programmed to calculate a white line approachsuppression steering reaction force that is based on a distance in whicha host vehicle is separated from either a left or a right white linewhen the host vehicle approaches the left or a right white line, andapplying a steering reaction force to a steering unit that receivessteering input from a driver, based on the white line approachsuppression steering reaction force, and upon determining that the otheramong the left and right white lines cannot be detected and a distancebetween the host vehicle and the left or right white line is decreasing,the white line approach suppression steering reaction force is limited.8. A steering control device comprising: a sensor for detecting a leftor a right white line of a traveling lane; and a controller programmedto calculate a white line approach suppression steering reaction forcethat is based on a distance between a host vehicle and the left or aright white line, when the host vehicle approaches either the left or aright white line, and the controller being further programmed to apply asteering reaction force to a steering unit that receives steering inputfrom a driver, based on the white line approach suppression steeringreaction force, and upon determining that the other among the left andright white lines cannot be detected and a distance between the hostvehicle and the left or right white line is decreasing, the white lineapproach suppression steering reaction force is limited.
 9. The steeringcontrol device according to claim 1, wherein the white line approachsuppression steering reaction force calculating unit is furtherprogrammed to calculate the white line approach suppression steeringreaction force such that the white line approach suppression steeringreaction force increases as the distance between the host vehicle andthe left or right white line decreases.
 10. The steering control deviceaccording to claim 9, wherein the limiting unit is further programmed tolimit the white line approach suppression steering reaction force, whenthe white line approach suppression steering reaction force is below aprevious value.
 11. The steering control device according to claim 9,wherein the limiting unit is further programmed to inhibit an increasein the white line approach suppression steering reaction force whilepermitting a decrease in the white line approach suppression steeringreaction force.
 12. The steering control device according to claim 9,wherein the limiting unit is further programmed to determine that theother among the left and right white lines is undetectable, when a statein which the other among the left and right white lines is undetectablehas continued for a predetermined time.
 13. The steering control deviceaccording to claim 9, wherein the steering reaction force control unitis further programmed to apply a steering reaction force to the steeringunit based on a command steering reaction force obtained by adding thewhite line approach suppression steering reaction force to the steeringreaction force that is obtained from a predetermined steering reactionforce characteristic, corresponding to a self-aligning torque that actson a turnable wheel.
 14. The steering control device according to claim9, wherein the steering unit is mechanically separated from a turningunit for turning a turnable wheel, the steering unit comprising: acurvature detection unit programmed to detect a curvature of a whiteline; and an offset amount calculation unit programmed to calculate anoffset amount that is greater as the detected curvature increases; thesteering reaction force control unit is further programmed to apply asteering reaction force to the steering unit based on a command steeringreaction force obtained from a self-aligning torque after being offset,which is obtained by subtracting the offset amount from theself-aligning torque that acts on the turnable wheel and a predeterminedsteering reaction force characteristic corresponding to theself-aligning torque, and the limiting unit being further programmed notto limit the offset amount, even when a determination has been made thatthe other among the left and right white lines is undetectable.
 15. Thesteering control device according to claim 2, wherein the limiting unitis further programmed to inhibit an increase in the white line approachsuppression steering reaction force while permitting a decrease in thewhite line approach suppression steering reaction force.
 16. Thesteering control device according to claim 2, wherein the limiting unitis further programmed to determine that the other among the left andright white lines is undetectable, when a state in which the other amongthe left and right white lines is undetectable has continued for apredetermined time.
 17. The steering control device according to claim2, wherein the steering reaction force control unit is furtherprogrammed to apply a steering reaction force to the steering unit basedon a command steering reaction force obtained by adding the white lineapproach suppression steering reaction force to the steering reactionforce that is obtained from a predetermined steering reaction forcecharacteristic, corresponding to a self-aligning torque that acts on aturnable wheel.
 18. The steering control device according to claim 2,wherein the steering unit is mechanically separated from a turning unitfor turning a turnable wheel, the steering unit comprising: a curvaturedetection unit programmed to detect a curvature of a white line; and anoffset amount calculation unit programmed to calculate an offset amountthat is greater as the detected curvature increases; the steeringreaction force control unit is further programmed to apply a steeringreaction force to the steering unit based on a command steering reactionforce obtained from a self-aligning torque after being offset, which isobtained by subtracting the offset amount from the self-aligning torquethat acts on the turnable wheel and a predetermined steering reactionforce characteristic corresponding to the self-aligning torque, and thelimiting unit being further programmed not to limit the offset amount,even when a determination has been made that the other among the leftand right white lines is undetectable.